The present invention relates to a signal selection circuit for selecting signals of predetermined levels out of a plurality of input signals. More specifically, the invention relates to a signal selection circuit which can be suitably employed for a high value selection circuit that selects signals of the highest level, which can be employed for a low value selection circuit that selects signals of the lowest level, and which can also be employed for a middle value selection circuit that selects signals of the middle level.
Control device employed in power plants (such as thermal power plants and nuclear power plants) and in chemical plants, create extensive adverse effects once they become defective. Therefore, very high reliability is required for such devices.
A doubled or tripled redundancy control device has heretofore been used significantly in such plants. The redundancy control device selects signals of the highest level, signals of the lowest level or signals of the middle level out of the signals (analog signals) produced by two or three sensors that detect the same state quantity depending upon the plants, and sends the selected signals to an actuator which directly controls an object that is to be controlled.
A low value selection circuit and a high value selection circuit have been disclosed in Japanese Patent Laid-Open No. 64251/1977, and middle value selection circuits have been disclosed in Japanese Patent Laid-Open No. 11638/1975 and in Japanese Utility Model Laid-Open No. 117001/1980. A middle value selection circuit has also been disclosed in U.S. Ser. No. 512,000 entitled "Middle Value Selection Circuit".